1. Field of Invention
Exemplary aspects of the present invention relate to an image forming apparatus, and more particularly to an image forming apparatus capable of preventing generation of a residual image and a failure in an image transfer.
2. Discussion of the Background
In recent years, downsizing, a reduction in weight, and an enhancement of processing speed are increasingly desired in an image forming apparatus. Even if the processing speed is enhanced, there are an increasing number of cases where a belt-type medium is not provided with a dedicated discharging member. The attenuation of residual charge of the belt-type medium is performed when the belt-type medium comes into contact with a grounded conductive spanned roller during a rotary operation thereof. The diameter of the grounded conductive roller is configured to be relatively large so that the contact time becomes longer, and thereby, the discharging effect is enhanced. Furthermore, in order to accommodate the attenuation characteristics of the belt-type medium, imaging conditions including a correction of transfer voltage conditions, for example, are addressed.
Japanese Patent Laid-Open Application Publication No. 2003-177610 proposes that the attenuation time constant of the potential of an intermediate transfer belt is configured to be between 0.01 seconds and 1000 seconds.
Though Japanese Patent Laid-Open Application Publication No. 2003-177610 proposes that the time constant is less than or equal to 600 seconds, preferably, less than or equal to 100 seconds, the attenuation speed may be relatively slow for the belt-type medium when the time constant is less than or equal to 600 seconds. Consequently, problems such as a residual image may be generated due to the residual potential. Furthermore, the attenuation of the charge of the charged belt is performed when coming into contact with a conductive portion. Therefore, when the charged belt is in contact with a metal roller or the like for a short period of time, a substantial attenuation may not be performed. In other words, when the time constant of the belt-type medium is 600 seconds, or even 100 seconds, the substantial discharging time may not be obtained. Consequently, a discharging mechanism is essentially required for the belt. In a case where the discharging mechanism is not provided, a correction of the transfer electric current may be necessary to accommodate the belt charge which varies depending on the use conditions.
Furthermore, in order to accommodate demands for downsizing, a reduction in weight, and an enhancement of processing speed, the diameter of the conductive spanned roller is configured to be relatively large. However, when the diameter of the conductive spanned roller is relatively large, the weight and the belt perimeter will increase, accordingly. As a result, the unit will become larger in size, and the cost may increase accordingly.
When an image formation is continuously performed while the residual potential is accumulated on the belt-type medium, problems such as the residual image and an image failure may be generated. Because the residual potential of the belt-type medium is not constant, the correction of the imaging conditions, such as transfer conditions, is difficult to perform. Therefore, transfer failure can easily occur.
Normally, a primary transfer voltage is applied by an elastic roller or the like at a downstream end of a nip. The surface resistivity of the rear surface affects the quality of the transfer image. When the surface resistivity thereof is relatively low, the charge migrates in a plane direction, and the charge distribution at the transfer nip is widened. Consequently, the belt potential at the beginning portion of the primary transfer increases, thereby increasing the electric field at a void area and causing image debris to be easily generated.
On the other hand, when the surface resistivity of the rear surface is relatively high, the charge migration in the plane direction decreases, and the charge distribution is narrowed. Consequently, the electric field at the void area of the beginning portion of the primary transfer is reduced, thereby making it possible to prevent image debris. In other words, when the surface resistivity of the belt-like medium is in a range between 1×1010 Ω and 5×1012 Ω, the charge migration in the plane direction may be reduced, and the effectiveness of the prevention of the image debris may be enhanced. However, the attenuation of an area in the vicinity of the contact area is reduced due to the charge flow. As a result, the efficiency of the belt discharge by the conductive roller is decreased. Furthermore, when the surface resistivity of the rear surface is relatively high, the discharge time, in which the conductive roller comes into contact with the belt-type medium, will become an issue.
When the surface resistivity of the rear surface of the belt-type medium is less than or equal to 1×1010 Ω, particularly less than or equal to 1×109 Ω, more charge flows in the plane direction so that an effective discharge may be performed. Accordingly, the discharge may be effectively performed on the roller of a small diameter which causes a short contact time. On the other hand, when the surface resistivity of the rear surface is more than or equal to 5×1012 Ω, discharge may occur after the primary transfer, thereby causing the image failure to easily occur.
The amount of the charge of the rear and front surfaces of the belt immediately after the primary transfer is not the same. The charge from the nip is supplied to the place from the nip end to the separating position to perform discharge. However, depending on the surface resistivity, the amount of the charge supply may vary. When the surface resistivity is more than or equal to 5×1012 Ωthe amount of the charge supplied from the nip is less so that discharge is difficult to perform. Thus, the electric field around the belt is unstable and may cause nearby devices to discharge.